over 1/2 of the u.s. population relies on it for its drinking-water supply. even more groundwater's used for irrigating agriculture, and its industrial use is growing every day. groundwater is valuable because it's plentiful and clean. there's about 50 times more water underground than in all the lakes and rivers on the earth's surface combined. in many areas, especially those with dry climates, groundwater is the most abundant and economical source of water available. because it's filtered as it passes through the soil, groundwater is usually less polluted than surface water. but this valuable resource is now being threatened. in some places, groundwater has been contaminated by industrial or agricultural pollution. in others, wells extract groundwater faster than it can be replenished. already, this has caused severe economic and health problems in several areas of the world. consequently, there's an increasingly important role for the geologists who study water movement underground and who can accurately predict the location and quantity of groundwater. "w